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RFC 8350

Experimental
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Alternate Tunnel Encapsulation for Data Frames in Control and Provisioning of Wireless Access Points (CAPWAP)

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Internet Engineering Task Force (IETF)                          R. Zhang
Request for Comments: 8350                                 China Telecom
Category: Experimental                                     R. Pazhyannur
ISSN: 2070-1721                                            S. Gundavelli
                                                                   Cisco
                                                                  Z. Cao
                                                                 H. Deng
                                                                   Z. Du
                                                                  Huawei
                                                              April 2018


           Alternate Tunnel Encapsulation for Data Frames in
      Control and Provisioning of Wireless Access Points (CAPWAP)

Abstract

   Control and Provisioning of Wireless Access Points (CAPWAP) is a
   protocol for encapsulating a station's data frames between the
   Wireless Transmission Point (WTP) and Access Controller (AC).
   Specifically, the station's IEEE 802.11 data frames can be either
   locally bridged or tunneled to the AC.  When tunneled, a CAPWAP Data
   Channel is used for tunneling.  In many deployments, encapsulating
   data frames to an entity other than the AC (for example, to an Access
   Router (AR)) is desirable.  Furthermore, it may also be desirable to
   use different tunnel encapsulation modes between the WTP and the
   Access Router.  This document defines an extension to the CAPWAP
   protocol that supports this capability and refers to it as alternate
   tunnel encapsulation.  The alternate tunnel encapsulation allows 1)
   the WTP to tunnel non-management data frames to an endpoint different
   from the AC and 2) the WTP to tunnel using one of many known
   encapsulation types, such as IP-IP, IP-GRE, or CAPWAP.  The WTP may
   advertise support for alternate tunnel encapsulation during the
   discovery and join process, and the AC may select one of the
   supported alternate tunnel encapsulation types while configuring the
   WTP.

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Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are candidates for any level of
   Internet Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8350.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Conventions Used in This Document . . . . . . . . . . . .   7
     1.2.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   7
     1.3.  History of the Document . . . . . . . . . . . . . . . . .   8
   2.  Alternate Tunnel Encapsulation Overview . . . . . . . . . . .   9
   3.  Extensions for CAPWAP Protocol Message Elements . . . . . . .  11
     3.1.  Supported Alternate Tunnel Encapsulations . . . . . . . .  11
     3.2.  Alternate Tunnel Encapsulations Type  . . . . . . . . . .  11
     3.3.  IEEE 802.11 WTP Alternate Tunnel Failure Indication . . .  12
   4.  Alternate Tunnel Types  . . . . . . . . . . . . . . . . . . .  13
     4.1.  CAPWAP-Based Alternate Tunnel . . . . . . . . . . . . . .  13
     4.2.  PMIPv6-Based Alternate Tunnel . . . . . . . . . . . . . .  14
     4.3.  GRE-Based Alternate Tunnel  . . . . . . . . . . . . . . .  15
   5.  Alternate Tunnel Information Elements . . . . . . . . . . . .  16
     5.1.  Access Router Information Elements  . . . . . . . . . . .  16
       5.1.1.  AR IPv4 List Element  . . . . . . . . . . . . . . . .  16
       5.1.2.  AR IPv6 List Element  . . . . . . . . . . . . . . . .  17
     5.2.  Tunnel DTLS Policy Element  . . . . . . . . . . . . . . .  17
     5.3.  IEEE 802.11 Tagging Mode Policy Element . . . . . . . . .  19
     5.4.  CAPWAP Transport Protocol Element . . . . . . . . . . . .  20
     5.5.  GRE Key Element . . . . . . . . . . . . . . . . . . . . .  22
     5.6.  IPv6 MTU Element  . . . . . . . . . . . . . . . . . . . .  23
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  24
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  25
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  25
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  25
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  27
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  28
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  28

1.  Introduction

   Service Providers are deploying very large Wi-Fi networks containing
   hundreds of thousands of Access Points (APs), which are referred to
   as Wireless Transmission Points (WTPs) in Control and Provisioning of
   Wireless Access Points (CAPWAP) terminology [RFC5415].  These
   networks are designed to carry traffic generated from mobile users.
   The volume in mobile user traffic is already very large and expected
   to continue growing rapidly.  As a result, operators are looking for
   scalable solutions that can meet the increasing demand.  The
   scalability requirement can be met by splitting the control/
   management plane from the data plane.  This enables the data plane to
   scale independent of the control/management plane.  This
   specification provides a way to enable such separation.

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   CAPWAP [RFC5415] [RFC5416] defines a tunnel mode that describes how
   the WTP handles the data plane (user traffic).  The following types
   are defined:

   o  Local Bridging: All data frames are locally bridged.

   o  IEEE 802.3 Tunnel: All data frames are tunneled to the Access
      Controller (AC) in IEEE 802.3 format.

   o  IEEE 802.11 Tunnel: All data frames are tunneled to the AC in IEEE
      802.11 format.

   Figure 1 describes a system with Local Bridging.  The AC is in a
   centralized location.  The data plane is locally bridged by the WTPs;
   this leads to a system with a centralized control plane and a
   distributed data plane.  This system has two benefits: 1) it reduces
   the scale requirement on the data traffic handling capability of the
   AC, and 2) it leads to more efficient/optimal routing of data traffic
   while maintaining centralized control/management.

                     Locally Bridged
             +-----+ Data Frames   +----------------+
             | WTP |===============|  Access Router |
             +-----+               +----------------+
                    \\
                     \\  CAPWAP Control Channel   +----------+
                       ++=========================|   AC     |
                      // CAPWAP Data Channel:     |          |
                     //  IEEE 802.11 Mgmt Traffic +----------+
                    //
             +-----+               +----------------+
             | WTP |============== |  Access Router |
             +-----+               +----------------+
                    Locally Bridged
                    Data Frames

            Figure 1: Centralized Control with Distributed Data

   The AC handles control of WTPs.  In addition, the AC also handles the
   IEEE 802.11 management traffic to/from the stations.  There is a
   CAPWAP Control and Data Channel between the WTP and the AC.  Note
   that even though there is no user traffic transported between the WTP
   and AC, there is still a CAPWAP Data Channel.  The CAPWAP Data
   Channel carries the IEEE 802.11 management traffic (like IEEE 802.11
   Action Frames).

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   Figure 2 shows a system where the tunnel mode is configured to tunnel
   data frames between the WTP and the AC using either the IEEE 802.3
   Tunnel or 802.11 Tunnel configurations.  Operators deploy this
   configuration when they need to tunnel the user traffic.  The
   tunneling requirement may be driven by the need to apply policy at
   the AC.  This requirement could be met in the locally bridged system
   (Figure 1) if the Access Router (AR) implemented the required policy.
   However, in many deployments, the operator managing the WTP is
   different than the operator managing the Access Router.  When the
   operators are different, the policy has to be enforced in a tunnel
   termination point in the WTP operator's network.

              +-----+
              | WTP |
              +-----+
                  \\
                    \\  CAPWAP Control Channel   +----------+
                      ++=========================|   AC     |
                     // CAPWAP Data Channel:     |          |
                    //  IEEE 802.11 Mgmt Traffic |          |
                   //   Data Frames              +----------+
                  //
              +-----+
              | WTP |
              +-----+

            Figure 2: Centralized Control and Centralized Data

   The key difference with the locally bridged system is that the data
   frames are tunneled to the AC instead of being locally bridged.
   There are two shortcomings with the system in Figure 2: 1) it does
   not allow the WTP to tunnel data frames to an endpoint different from
   the AC, and 2) it does not allow the WTP to tunnel data frames using
   any encapsulation other than CAPWAP (as specified in Section 4.4.2 of
   [RFC5415]).

   Figure 3 shows a system where the WTP tunnels data frames to an
   alternate entity different from the AC.  The WTP also uses an
   alternate tunnel encapsulation such as Layer 2 Tunneling Protocol
   (L2TP), L2TPv3, IP-in-IP, IP/GRE, etc.  This enables 1) independent
   scaling of data plane and 2) leveraging of commonly used tunnel
   encapsulations such as L2TP, GRE, etc.

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          Alternate Tunnel to AR (L2TPv3, IP-IP, CAPWAP, etc.)
                       _________
         +-----+      (         )              +-----------------+
         | WTP |======+Internet +==============|Access Router(AR)|
         +-----+      (_________)              +-----------------+
               \\      ________  CAPWAP Control
                \\    (        ) Channel                +--------+
                   ++=+Internet+========================|   AC   |
                  //  (________)CAPWAP Data Channel:    +--------+
                 //             IEEE 802.11 Mgmt Traffic
                //   _________
         +-----+    (         )                +----------------+
         | WTP |====+Internet +================|  Access Router |
         +-----+    (_________)                +----------------+
          Alternate Tunnel to AR (L2TPv3, IP-in-IP, CAPWAP, etc.)

      Figure 3: Centralized Control with an Alternate Tunnel for Data

   The WTP may support widely used encapsulation types such as L2TP,
   L2TPv3, IP-in-IP, IP/GRE, etc.  The WTP advertises the different
   alternate tunnel encapsulation types it can support.  The AC
   configures one of the advertised types.  As is shown in Figure 3,
   there is a CAPWAP Control and Data Channel between the WTP and AC.
   The CAPWAP Data Channel carries the stations' management traffic, as
   in the case of the locally bridged system.  The main reason to
   maintain a CAPWAP Data Channel is to maintain similarity with the
   locally bridged system.  The WTP maintains three tunnels: CAPWAP
   Control, CAPWAP Data, and another alternate tunnel for the data
   frames.  The data frames are transported by an alternate tunnel
   between the WTP and a tunnel termination point, such as an Access
   Router.  This specification describes how the alternate tunnel can be
   established.  The specification defines message elements for the WTP
   to advertise support for alternate tunnel encapsulation, for the AC
   to configure alternate tunnel encapsulation, and for the WTP to
   report failure of the alternate tunnel.

   The alternate tunnel encapsulation also supports the third-party WLAN
   service provider scenario (i.e., Virtual Network Operator (VNO)).
   Under this scenario, the WLAN provider owns the WTP and AC resources
   while the VNOs can rent the WTP resources from the WLAN provider for
   network access.  The AC belonging to the WLAN service provider
   manages the WTPs in the centralized mode.

   As shown in Figure 4, VNO 1 and VNO 2 don't possess the network
   access resources; however, they provide services by acquiring
   resources from the WLAN provider.  Since a WTP is capable of
   supporting up to 16 Service Set Identifiers (SSIDs), the WLAN
   provider may provide network access service for different providers

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   with different SSIDs.  For example, SSID1 is advertised by the WTP
   for VNO 1 while SSID2 is advertised by the WTP for VNO 2.  Therefore,
   the data traffic from the user can be directly steered to the
   corresponding Access Router of the VNO who owns that user.  As is
   shown in Figure 4, AC can notify multiple AR addresses for load
   balancing or redundancy.

                                     +----+
                                     | AC |
                                     +--+-+
                          CAPWAP-CTL    |
                      +-----------------+
                      |   CAPWAP-DATA: IEEE 802.11 Mgmt Traffic
                      |
         WLAN Provider|                            VNO 1
                +-----+   CAPWAP-DATA (SSID1)    +---------------+
         SSID1  | WTP +--------------------------|Access Router 1|
         SSID2  +--+-++                          +---------------+
                   | |
                   | |                             VNO 1
                   | |    GRE-DATA (SSID1)       +---------------+
                   | +---------------------------|Access Router 2|
                   |                             +---------------+
                   |
                   |                               VNO 2
                   |      CAPWAP-DATA (SSID2)    +---------------+
                   +-----------------------------|Access Router 3|
                                                 +---------------+

                Figure 4: Third-Party WLAN Service Provider

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.2.  Terminology

   Station (STA): A device that contains an IEEE 802.11-conformant
   Medium Access Control (MAC) and Physical layer (PHY) interface to the
   Wireless Medium (WM).

   Access Controller (AC): The network entity that provides WTP access
   to the network infrastructure in the data plane, control plane,
   management plane, or a combination therein.

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   Access Router (AR): A specialized router usually residing at the edge
   or boundary of a network.  This router ensures the connectivity of
   its network with external networks, a wide area network, or the
   Internet.

   Wireless Termination Point (WTP): The physical or network entity that
   contains a Radio Frequency (RF) antenna and wireless Physical layer
   (PHY) to transmit and receive station traffic for wireless access
   networks.

   CAPWAP Control Channel: A bidirectional flow defined by the AC IP
   Address, WTP IP Address, AC control port, WTP control port, and the
   transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Control
   packets are sent and received.

   CAPWAP Data Channel: A bidirectional flow defined by the AC IP
   Address, WTP IP Address, AC data port, WTP data port, and the
   transport-layer protocol (UDP or UDP-Lite) over which CAPWAP Data
   packets are sent and received.  In certain WTP modes, the CAPWAP Data
   Channel only transports IEEE 802.11 management frames and not the
   data plane (user traffic).

1.3.  History of the Document

   This document was started to accommodate Service Providers' need of a
   more flexible deployment mode with alternative tunnels [RFC7494].
   Experiments and tests have been done for this alternate tunnel
   network infrastructure.  However important, the deployment of
   relevant technology is yet to be completed.  This Experimental
   document is intended to serve as an archival record for any future
   work on the operational and deployment requirements.

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2.  Alternate Tunnel Encapsulation Overview

           +-+-+-+-+-+-+                             +-+-+-+-+-+-+
           |    WTP    |                             |    AC     |
           +-+-+-+-+-+-+                             +-+-+-+-+-+-+
                 |Join Request [ Supported Alternate       |
                 |       Tunnel Encapsulations ]           |
                 |---------------------------------------->|
                 |                                         |
                 |Join Response                            |
                 |<----------------------------------------|
                 |                                         |
                 |IEEE 802.11 WLAN Configuration Request [ |
                 | IEEE 802.11 Add WLAN,                   |
                 | Alternate Tunnel Encapsulation (        |
                 |   Tunnel Type, Tunnel Info Element)     |
                 | ]                                       |
                 |<----------------------------------------|
                 |                                         |
                 |                                         |
            +-+-+-+-+-+-+                                  |
            | Setup     |                                  |
            | Alternate |                                  |
            | Tunnel    |                                  |
            +-+-+-+-+-+-+                                  |
                 |IEEE 802.11 WLAN Configuration Response  |
                 |[ Alternate Tunnel Encapsulation (       |
                 |   Tunnel Type, Tunnel Info Element) ]   |
                 |---------------------------------------->|
                 |                                         |
            +-+-+-+-+-+-+                                  |
            | Tunnel    |                                  |
            | Failure   |                                  |
            +-+-+-+-+-+-+                                  |
                 |WTP Alternate Tunnel Failure Indication  |
                 |(Report Failure (AR Address(es)))        |
                 |---------------------------------------->|
                 |                                         |
         +-+-+-+-+-+-+-+                                   |
         | Tunnel      |                                   |
         | Established |                                   |
         +-+-+-+-+-+-+-+                                   |
                 |WTP Alternate Tunnel Failure Indication  |
                 |(Report Clearing Failure)                |
                 |---------------------------------------->|
                 |                                         |

                  Figure 5: Setup of an Alternate Tunnel

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   The above example describes how the alternate tunnel encapsulation
   may be established.  When the WTP joins the AC, it should indicate
   its alternate tunnel encapsulation capability.  The AC determines
   whether an alternate tunnel configuration is required.  If an
   appropriate alternate tunnel type is selected, then the AC provides
   the Alternate Tunnel Encapsulations Type message element containing
   the tunnel type and a tunnel-specific information element.  The
   tunnel-specific information element, for example, may contain
   information like the IP address of the tunnel termination point.  The
   WTP sets up the alternate tunnel using the Alternate Tunnel
   Encapsulations Type message element.

   Since an AC can configure a WTP with more than one AR available for
   the WTP to establish the data tunnel(s) for user traffic, it may be
   useful for the WTP to communicate the selected AR.  To enable this,
   the IEEE 802.11 WLAN Configuration Response may carry the Alternate
   Tunnel Encapsulations Type message element containing the AR list
   element corresponding to the selected AR as shown in Figure 5.

   On detecting a tunnel failure, the WTP SHALL forward data frames to
   the AC and discard the frames.  In addition, the WTP may dissociate
   existing clients and refuse association requests from new clients.
   Depending on the implementation and deployment scenario, the AC may
   choose to reconfigure the WLAN (on the WTP) to a Local Bridging mode
   or to tunnel frames to the AC.  When the WTP detects an alternate
   tunnel failure, the WTP informs the AC using a message element, IEEE
   802.11 WTP Alternate Tunnel Failure Indication (defined in
   Section 3.3).  It MAY be carried in the WTP Event Request message,
   which is defined in [RFC5415].

   The WTP also needs to notify the AC of which AR(s) are unavailable.
   Particularly, in the VNO scenario, the AC of the WLAN service
   provider needs to maintain the association of the AR addresses of the
   VNOs and SSIDs and provide this information to the WTP for the
   purpose of load balancing or master-slave mode.

   The message element has a Status field that indicates whether the
   message is reporting a failure or clearing the previously reported
   failure.

   For the case where an AC is unreachable but the tunnel endpoint is
   still reachable, the WTP behavior is up to the implementation.  For
   example, the WTP could choose to either tear down the alternate
   tunnel or let the existing user's traffic continue to be tunneled.

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3.  Extensions for CAPWAP Protocol Message Elements

3.1.  Supported Alternate Tunnel Encapsulations

   This message element is sent by a WTP to communicate its capability
   to support alternate tunnel encapsulations.  The message element
   contains the following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Tunnel-Type 1            |      Tunnel-Type 2            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |            ...                |      Tunnel-Type N            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 6: Supported Alternate Tunnel Encapsulations

   o  Type: 54 for Supported Alternate Tunnel Encapsulations Type

   o  Length: The length in bytes; two bytes for each Alternative
      Tunnel-Type that is included

   o  Tunnel-Type: This is identified by the value defined in
      Section 3.2.  There may be one or more Tunnel-Types, as is shown
      in Figure 6.

3.2.  Alternate Tunnel Encapsulations Type

   This message element can be sent by the AC, allows the AC to select
   the alternate tunnel encapsulation, and may be provided along with
   the IEEE 802.11 Add WLAN message element.  When the message element
   is present, the following fields of the IEEE 802.11 Add WLAN element
   SHALL be set as follows: MAC mode is set to 0 (Local MAC), and Tunnel
   Mode is set to 0 (Local Bridging).  Besides, the message element can
   also be sent by the WTP to communicate the selected AR(s).

   The message element contains the following fields:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      Tunnel-Type              |  Info Element Length          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Info Element
     +-+-+-+-+-+-+-+-+-+

              Figure 7: Alternate Tunnel Encapsulations Type

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   o  Type: 55 for Alternate Tunnel Encapsulations Type

   o  Length: > 4

   o  Tunnel-Type: The Tunnel-Type is specified by a 2-byte value.  This
      specification defines the values from 0 to 6 as given below.  The
      remaining values are reserved for future use.

      *  0: CAPWAP.  This refers to a CAPWAP Data Channel described in
         [RFC5415] and [RFC5416].

      *  1: L2TP.  This refers to tunnel encapsulation described in
         [RFC2661].

      *  2: L2TPv3.  This refers to tunnel encapsulation described in
         [RFC3931].

      *  3: IP-in-IP.  This refers to tunnel encapsulation described in
         [RFC2003].

      *  4: PMIPv6-UDP.  This refers to the UDP encapsulation mode for
         Proxy Mobile IPv6 (PMIPv6) described in [RFC5844].  This
         encapsulation mode is the basic encapsulation mode and does not
         include the TLV header specified in Section 7.2 of [RFC5845].

      *  5: GRE.  This refers to GRE tunnel encapsulation as described
         in [RFC2784].

      *  6: GTPv1-U.  This refers to the GPRS Tunnelling Protocol (GTP)
         User Plane mode as described in [TS.3GPP.29.281].

   o  Info Element: This field contains tunnel-specific configuration
      parameters to enable the WTP to set up the alternate tunnel.  This
      specification provides details for this element for CAPWAP,
      PMIPv6, and GRE.  This specification reserves the tunnel type
      values for the key tunnel types and defines the most common
      message elements.  It is anticipated that message elements for the
      other protocols (like L2TPv3) will be defined in other
      specifications in the future.

3.3.  IEEE 802.11 WTP Alternate Tunnel Failure Indication

   The WTP MAY include the Alternate Tunnel Failure Indication message
   in a WTP Event Request message to inform the AC about the status of
   the alternate tunnel.  For the case where the WTP establishes data
   tunnels with multiple ARs (e.g., under a VNO scenario), the WTP needs
   to notify the AC of which AR(s) are unavailable.  The message element
   contains the following fields:

Top      ToC       Page 13 
      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |      WLAN ID  |     Status    |         Reserved              |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .              Access Router Information Element                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Figure 8: IEEE 802.11 WTP Alternate Tunnel Failure Indication

   o  Type: 1062 for IEEE 802.11 WTP Alternate Tunnel Failure Indication

   o  Length: > 4

   o  WLAN ID: An 8-bit value specifying the WLAN Identifier.  The value
      MUST be between 1 and 16.

   o  Status: An 8-bit boolean indicating whether the radio failure is
      being reported or cleared.  A value of 0 is used to clear the
      event, while a value of 1 is used to report the event.

   o  Reserved: MUST be set to a value of 0 and MUST be ignored by the
      receiver.

   o  Access Router Information Element: The IPv4 or IPv6 address of the
      Access Router that terminates the alternate tunnel.  The Access
      Router Information Elements allow the WTP to notify the AC of
      which AR(s) are unavailable.

4.  Alternate Tunnel Types

4.1.  CAPWAP-Based Alternate Tunnel

   If the CAPWAP encapsulation is selected by the AC and configured by
   the AC to the WTP, the Info Element field defined in Section 3.2
   SHOULD contain the following information:

   o  Access Router Information: The IPv4 or IPv6 address of the Access
      Router for the alternate tunnel.

   o  Tunnel DTLS Policy: The CAPWAP protocol allows optional protection
      of data packets using DTLS.  Use of data packet protection on a
      WTP is not mandatory but is determined by the associated AC
      policy.  (This is consistent with the WTP behavior described in
      [RFC5415].)

Top      ToC       Page 14 
   o  IEEE 802.11 Tagging Mode Policy: It is used to specify how the
      CAPWAP Data Channel packets are to be tagged for QoS purposes (see
      [RFC5416] for more details).

   o  CAPWAP Transport Protocol: The CAPWAP protocol supports both UDP
      and UDP-Lite (see [RFC3828]).  When run over IPv4, UDP is used for
      the CAPWAP Data Channels.  When run over IPv6, the CAPWAP Data
      Channel may use either UDP or UDP-Lite.

   The message element structure for CAPWAP encapsulation is shown in
   Figure 9:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Tunnel-Type=0             |   Info Element Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .              Access Router Information Element                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .              Tunnel DTLS Policy Element                       .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .             IEEE 802.11 Tagging Mode Policy Element           .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .             CAPWAP Transport Protocol Element                 .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 9: Alternate Tunnel Encapsulation - CAPWAP

4.2.  PMIPv6-Based Alternate Tunnel

   A user plane based on PMIPv6 (defined in [RFC5213]) can also be used
   as an alternate tunnel encapsulation between the WTP and the AR.  In
   this scenario, a WTP acts as the Mobile Access Gateway (MAG) function
   that manages the mobility-related signaling for a station that is
   attached to the WTP IEEE 802.11 radio access.  The Local Mobility
   Anchor (LMA) function is at the AR.  If PMIPv6 UDP encapsulation is
   selected by the AC and configured by the AC to a WTP, the Info
   Element field defined in Section 3.2 SHOULD contain the following
   information:

   o  Access Router (acting as LMA) Information: IPv4 or IPv6 address
      for the alternate tunnel endpoint.

Top      ToC       Page 15 
   The message element structure for PMIPv6 encapsulation is shown in
   Figure 10:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Tunnel-Type=4             |   Info Element Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                 Access Router Information Element             .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            Figure 10: Alternate Tunnel Encapsulation - PMIPv6

4.3.  GRE-Based Alternate Tunnel

   A user plane based on Generic Routing Encapsulation (defined in
   [RFC2784]) can also be used as an alternate tunnel encapsulation
   between the WTP and the AR.  In this scenario, a WTP and the Access
   Router represent the two endpoints of the GRE tunnel.  If GRE is
   selected by the AC and configured by the AC to a WTP, the Info
   Element field defined in Section 3.2 SHOULD contain the following
   information:

   o  Access Router Information: The IPv4 or IPv6 address for the
      alternate tunnel endpoint.

   o  GRE Key Information: The Key field is intended to be used for
      identifying an individual traffic flow within a tunnel [RFC2890].

   The message element structure for GRE is shown in Figure 11:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     Tunnel-Type=5             |   Info Element Length         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .              Access Router Information Element                .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     .                    GRE Key Element                            .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 11: Alternate Tunnel Encapsulation - GRE


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